It is known in the art to provide ceramic materials for use in applications requiring high thermal stability. One of the major drawbacks with ceramic materials however, is that they are not easily processed using conventional techniques. Ceramics are generally known to have high softening temperatures, and therefore, are not as easily shaped as compared to organic polymers having lower softening temperatures, which are often melt processed. Therefore, ceramic materials are usually machined or otherwise mechanically shaped before they are employed. Ceramic, metal and some polymer processing requires extreme conditions (temperature and pressure) or complicated equipment for processing, and therefore, experience a higher scrap rate.
Because of the processability problems with ceramic materials, it is known to process ceramic precursors to a desired shape, and then pyrolyze the precursor to form a ceramic material. However, many ceramics are brittle and therefore, not useful for applications where structural integrity is required.
Silicone based laminating materials are also known in the art. However, these materials, including silicone based ceramics, suffer the same poor processability characteristics of other ceramics. In addition, silicone laminating materials are also often brittle and subject to delamination. Further, silicone laminating materials generally are known to have limited thermal protection characteristics and limited resistance to thermal shock.
Thermal shock is generally understood to mean the stresses induced when a material is suddenly exposed to a higher temperature than its pre-exposed temperature. Many insulating materials are capable of withstanding high temperatures, including many ceramics. Hence, ceramics are often employed to make articles requiring at least a certain degree of heat resistance. If the temperature rise is gradual, many ceramics will effectively function as an insulator.
However, when the surface of an insulating material is subjected to a sudden temperature rise over a short period of time, such as from a substantially instantaneously applied flame or the like, the surface of the material will expand due to the temperature rise. The core of the material however, often remains at the ambient temperature, at least for a sufficient amount of time such that it will not expand as the surface is expanding. The effect of this temperature gradient within the materials, is that the material is stressed due to the unequal expansion rates, and cracks or the like can occur therein if the stresses are greater than the strength of the material. Thermally induced stresses can be represented by the following formula: EQU Thermal stress=.alpha.L.DELTA.T=.DELTA..epsilon..EPSILON.
where .alpha. is the linear thermal expansion coefficient of the material; L is the finite element length; .DELTA.T is the thermal gradient across the material; .DELTA..epsilon. is the thermal strain; and, .EPSILON. is the modulus of the material.
Because of the sudden exposure to an increased temperature, it is common to refer to a material's ability to withstand a sudden temperature increase as its resistance to "thermal shock". Monolithic materials which are known to crack upon sudden exposure to high temperatures include silica, quartz, alumina, zirconia and graphite.
Phenolic resin-based composites are known to provide high temperature resistance due to their ability to ablate, i.e., produce an insulating char. However, the char has poor mechanical characteristics and these composites have only limited uses. Carbon/carbon composites also have good thermal stability, but are known to have poor resistance to oxidative atmospheres and are difficult to fabricate. Ceramic-based materials have good thermal stability, but as discussed above, are difficult to process and have poor resistance to thermal shock.
A need exists therefore, for a processable material which possesses high thermal resistance properties, good structural properties, resistance to thermal shock and which is processable by conventional techniques.